libvirt: fix maxphysaddr passthrough dom parsing

If `hw:maxphysaddr_mode` is set to `passthrough`, the generated XML
doesn't contain a `bits` attribute. Our
`LibvirtConfigCPUMaxPhysAddr.parse_dom()` assumed `bits` was always set
and the tests only tested parsing the XML for the `emulate` mode.

Closes-Bug: #2099663
Change-Id: Ic16561dfb38612ac46c2148f2847006f6890940e

nova/tests/unit/virt/libvirt/test_config.py

@@ -547,7 +547,7 @@ class LibvirtConfigCPUTest(LibvirtConfigBaseTest):
             </cpu>
         """)
 
-    def test_parse_dom(self):
+    def test_parse_dom_emulate(self):
         xml = """
             <cpu>
               <maxphysaddr mode='emulate' bits='42'/>
@@ -560,6 +560,20 @@ class LibvirtConfigCPUTest(LibvirtConfigBaseTest):
         self.assertEqual("emulate", obj.maxphysaddr.mode)
         self.assertEqual(42, obj.maxphysaddr.bits)
 
+    def test_parse_dom_passthrough(self):
+        """Passthrough mode has no "bits" attribute"""
+        xml = """
+            <cpu>
+              <maxphysaddr mode='passthrough'/>
+            </cpu>
+        """
+        xmldoc = etree.fromstring(xml)
+        obj = config.LibvirtConfigCPU()
+        obj.parse_dom(xmldoc)
+
+        self.assertEqual("passthrough", obj.maxphysaddr.mode)
+        self.assertIsNone(obj.maxphysaddr.bits)
+
 
 class LibvirtConfigGuestCPUTest(LibvirtConfigBaseTest):
 

nova/virt/libvirt/config.py

@@ -851,7 +851,8 @@ class LibvirtConfigCPUMaxPhysAddr(LibvirtConfigObject):
         super(LibvirtConfigCPUMaxPhysAddr, self).parse_dom(xmldoc)
 
         self.mode = xmldoc.get("mode")
-        self.bits = int(xmldoc.get("bits"))
+        if xmldoc.get("bits") is not None:
+            self.bits = int(xmldoc.get("bits"))
 
     def format_dom(self):
         m = super(LibvirtConfigCPUMaxPhysAddr, self).format_dom()